Typical current digital communication systems often use non-constant envelope modulation schemes, e.g. the new system EDGE using 3π/8-8PSK modulation. This means that some part of the information lies in the amplitude (envelope) of the transmitted signal and some part lies in the phase of the transmitted signal. In other words, this is a combination of Amplitude Modulation (AM) and Phase Modulation (PM).
To deal with amplitude modulation, an output Power Amplifier (PA) in the radio transmitter has to be linear, i.e. the relationship between the output power of the PA (Pout,PA) and the input power of the PA (Pin,PA) has to be linear for all possible power levels. Otherwise the result will be AM-to-AM distortion, i.e. the gain of the PA changes with the input amplitude.
To deal with the phase modulation, the phase-shift (Δφ) through the PA has to be constant for all possible power levels. Otherwise the result will be AM-to-PM distortion, i.e. the phase-shift of the PA changes with the input amplitude.
The consequences of using a PA with non-constant gain and/or non-constant phase-shift, will be amplitude distortion and/or phase distortion in the transmitted signal. This distortion leads to spectrum broadening, which results in an increased adjacent channel disturbance. The amplitude/phase distortion (vector distortion) in the transmitter also affects the performance of the communications system. For example, an increased BER (Bit Error Rate) in the communication system, will lead to a decreased signal quality (e.g. degraded audio quality in a voice application).
Therefore, linearity is crucial for a transmitter used in a digital modulation system with non-constant amplitude modulation. Moreover, high linearity requirements often lead to poor power efficiency. To attain good linearity and good power efficiency, some linearization method and/or some efficiency enhancement method are often used. A problem that often arises is then poor time alignment between the “information parameters” (or “information components”), i.e. gain and phase (polar representation), alternatively I and Q (cartesian representation).
There are several known ways to attain linearity and/or power efficiency in RF (Radio Frequency) transmitters for digital modulation systems with non-constant amplitude modulation, for example:                Polar Loop Feedback        Cartesian Loop Feedback        Predistortion        Adaptive Baseband Predistortion        Feed-forward        Envelope Elimination and Restoration        Combining two power amplifiersThe methods can be divided in three categories:1) How the modulation is generated:        Cartesian modulation, i.e. in-phase (I) and quadrature (Q)        Polar modulation (e.g. Envelope Elimination and Restoration), i.e. the signal is divided into amplitude information (r) and phase information (φ)2) Whether or not the method uses feedback        Examples of methods using feedback: Polar loop feedback, Cartesian loop feedback, Adaptive baseband predistortion        Examples of methods not using feedback: Predistortion, Feedforward, Envelope elimination and restoration, combination of 2 non-linear signals paths (e.g. LINC or CALLUM). For example, see D C Cox, “Linear amplification with non-linear components”, IEEE Transactions on Communications, Vol 22, No. 12, pp 1942–1945, Dec 1974; and A. Bateman, “The combined analogue locked loop universal modulator (Callum), proceedings of the 42nd IEEE Vehicular Technical Conference, May 1992, pp 759–764.3) How the feedback signal path, if any, is implemented        I/Q-demodulator (I/Q-feedback),        Amplitude feedback only        Phase feedback only        Both amplitude and phase-feedback        